One example of a communications link for use in connection with data terminal equipment (DTE) is an asynchronous data service proposed for the INMARSAT-B (TM) or INMARSAT-M (TM) satellite communication systems, as described for example in chapters 12 and 14 of "Satellite Communications: Principles and Applications" by Calcutt and Tetley, First Edition, published by Edward Arnold.
The overall layout of the satellite communication system, when used for data communications, is shown in FIG. 1. A mobile DTE 2 is connected via an RS232 interface to a mobile modem interface unit (MIU) 4. The mobile MIU 4 simulates a modem, so that off-the-shelf communication software may be used in the mobile DTE 2. The MIU 4 provides an interface to a mobile earth station (MES) 6 which allows communication via a satellite 8 to a fixed or land earth station (LES) 10. The LES 10 is connected to a fixed MIU 12 which interfaces the satellite link to a public switched telephone network (PSTN) 14. A fixed DTE 18 is connected to the PSTN 14 through a modem 16 of standard type.
Under good conditions, the PSTN 14 should be capable of supporting bit rates as high as 28,800 bits per second or more. However, the INMARSAT-M satellite link supports a maximum data rate of 2400 bits per second, while the INMARSAT-B system supports a maximum of 9600 bits per second. If an acceptable bit error rate for data communications is to be provided over the satellite links, the data transfer rate must be limited, and so the PSTN link is limited to the data rate available over the satellite link.
The document EP-A-0566407 discloses an interface between a local area network and an RF segment of a terrestrial cellular system, in which compressed voice traffic from a mobile terminal is expanded for transmission to a PSTN, but remains in compressed form if the voice traffic is intended for reception by another mobile terminal, to avoid double coding of speech. However, such an interface does not allow a user to communicate using either compressed or uncompressed data over the same network.